Thermomagnetic recording process

ABSTRACT

A recording process is provided for a thermomagnetic recording system wherein recording is by a combination of heating and magnetization of a medium which consists largely of cobalt in hexagonal form. The medium is plated from a cobalt electrolite containing a small amount of phosphorous at a pH of from 5.5 to 6.6. A small amount of nickel may also be present. The resulting recording medium has strongly temperature dependent coercivity characteristics at temperatures well below the Curie point and the ratio of coercivity at 15* C. to that at 150* C. is at least two.

United States Patent- [72] Inventors lrving W. Wolf Palo Alto, Calif.;David Treves, Rehovoth, Israel; Nathan Ballard, Santa Clara, Calif. [21]Appl. No. 739,602 [22] Filed May 13, 1968 [45] Patented Oct. 5, 1971[73] Assignee Ampex Corporation Redwood City, Calif.

[54] THERMOMAGNETIC RECORDING PROCESS 4 Claims, 1 Drawing Fig.

[52] US. Cl 346/74, 117/238, 179/1002, 204/43 [51] lnt.Cl ..G01d 15/12,C23b 5/32 [50] Field of Search 346/74 M,

74 MT, 74 MP; 179/1002 CF, 100.2 CR; ll7/235, 238; 204/43; 250/65. 1;340/174 TF, 174

[56] References Cited UNITED STATES PATENTS 3,364,496 H1968 Greiner etal 346/74 3,463,708 8/1969 Grant 204/43 Primary Examiner-Bernard KonickAssistant ExaminerGary M. Hoffman AttorneyRobert G. Clay ABSTRACT: Arecording process is provided for a thermomagnetic recording systemwherein recording is by a combination of heating and magnetization of amedium which consists largely of cobalt in hexagonal form. The medium isplated from a cobalt electrolite containing a small amount ofphosphorous at a pH of from 5.5 to 6.6. A small amount of nickel mayalso be present. The resulting recording medium has strongly temperaturedependent coercivity characteristics at temperatures well below theCurie point and the ratio'of coercivity at 15C. to that at 150 C. is atleast two.

TEMPERATURE c PATENTEDUBT 5|97l 3511,141

200 E n N.

I00 I u w TEMPERATURE C INVENTORS mvms w. WOLF, DAVID TREVES.& BY NATHANBALLARD AT ORNE Y 1 THERMOMAGNETIC RECORDING PROCESS BACKGROUND OF THEINVENTION 1. Field of the Invention A Thermomagnetic recording process.

2. Description of the Prior Art The materials heretofore available fortherrnomagnetic recording were relatively inefficient and requiredhigh-recording temperatures. The present invention difiers from theprior art in the use of materials having a high ratio of coercivity atnon-nal temperatures to relatively low elevated temperatures, i.e., wellbelow the Curie point.

SUMMARY OF THE INVENTION Various recording techniques utilizing acombination of heating and magnetism have been known in the past. Onetypical system utilizes an electron beam to locally heat a magnetizedfilm to the Curie temperature, causing the coercivity of the heated spotof film to decrease, permitting the reversal of magnetism therein by arelatively weak field. Ordinarily temperatures as high as l,000 C. arerequired. Typical temperature thermowrite devices are described in U.S.Pat. Nos. 3,176,278 and 2,915,594. Another thermal system utilizesgarnet materials to accomplish thermal writing at a temperature near thecompensation point at which the average magnetic moment of the materialsgoes to zero. Other proposed systems utilize the combination ofthermally induced stress and magnetostriction effects to providerecording while others employ radiation with ultraviolet light to changethe magnetic moment of gadolinium iron garnets.

In pending application, Ser. No. 622,795 now US. Pat. No. 3,521,294 animproved magnetothermal recording method is disclosed wherein writing isachieved by locally heating an anomalous medium while simultaneouslymagnetizing it, said heating being at a temperature substantially belowthat of the Curie temperature region.

The present invention provides improved materials'for such a lowtemperature thermal recording system which operates efiiciently attemperatures as low as 150 to 200 C. while prior art materials utilizingthe Curie effect must operate at temperatures on the order of 1,000" C.Because of the low temperature involved and the short heating time, theuse of plastic substrates is entirely practical.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE of the drawing is agraph showing the decrease in coercivity of a material prepared inaccordance with example 1 of the present invention. It shows that thematerial has a coercivity of about 450 Oe at room temperature and thatthe coercivity has fallen to about one-fourth of its former value at 150C. and to about one-sixth at 200 C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Copending patent applicationSer.'No. 622,795 describes in detail the utilization of materials havinganomalous thermomagnetic properties. In general, the method consists ofproviding a heat-generating field such as an electric heater or anelectron beam source which is focused on a small portion of therecording medium. Simultaneously, a weak magnetic field is applied tothe same portion of the recording medium. Either the heat beam or themagnetic field, or both, can be modulated to record the desiredinformation on the recording medium.

In accordance with the present invention, it has been found that ahighly effective low-temperature thermal writing process can be providedby electroplating a film containing predominately cobalt with a smallamount of phosphorus and optionally a small amount of nickel. If suchplating were conducted in accordance with normal practice, i.e., using arelatively low pH, ordinary cubic cobalt would be deposited and theanomalous behavior desired, i.e., a highly sensitive temperaturedependent coercivity, would not be achieved. In accordance with thepresent invention, it has been found that if the plating is conducted ata high pH, i.e., from 5.5 to 6.6 or preferably from 6 to 6.6, cobalt isnot deposited in the cubic form, but is deposited in a hexagonal form,yielding a recording medium having the desired temperature-sensitivecoercivi ty. If the pH of the plating solution is below 5.5 the cubicform is deposited, defeating the purpose of the present invention while6.6 is given as an upper limit only because at a higher pH there willnormally be precipitation of the salts from the plating solution; thisis ordinarily undesirable since any solid, suspended matter in theplating solution might interfere with the deposit of a smooth, evencoating.

By following the technique of the present invention, it is easy toachieve ratios of coercivity of 2:1 and even as high as 4:1 as is shownin example I in the drawing. By ratio, is meant the ratio of thecoercivity of the material at about room temperature, i.e., 15 C. and ata temperature of 150 C. In addition, the coercivities of the materialsare well over at room temperature, yielding a satisfactory output andrendering them more or less immune to print through self-demagnetizingeffects at workable densities or changes in magnetization due to small,stray magnetic fields. The materials utilized in the process of thepresent invention also have an excellent squareness or remanence ratio(Br/Bs,) as is shown by the data wherein all of the materials of thepresent invention have a squareness of better than 0.8.

Various salts can be used in the plating bath and, as the examplesshows, the effects are not dependent upon the anions in the bath. Thus,there can be mixtures of sulfate and chloride, all sulfate, allchloride, or sulfamate, for example without substantially affecting theprocess of the present invention.

The plating conditions can vary over wide limits. Although it isnormally preferred to operate at a current density of about 100 ma. persquare inch, current densities as low as 5 ma. may be employed althoughat these low-current densities plating takes place so slowly that it isnot economically desirable. The current density can go as high as 500ma. per square inch but at higher densities the coating tends to becomegrainy and therefore less useful.

The change in coercivity upon heating is almost completely reversible asis shown in the working examples. Thus, when the plating of the presentinvention was heated to C. and held at this temperature for 10 minutes,the coercivity returned to almost its former value when the material wascooled to 15 C.

The concentration of the materials on the plating bath is ordinarily notcritical so long as the concentration is not made so great or the pHraised to the point where materials precipitate from the bath.

The ratio of phosphorus (as hypophosphite) to cobalt in the bath canvary from about 0.08/100 to 2.3/100 by weight and the concentration innickel can vary from 0/100 to 10/100. The metallic ions do not plate outin direct proportion to their concentration in the bath, the proportionsbeing approximately 1.5 to l in the case of phosphorus and approximately4 to l in the case of nickel. Therefore with respect to the platingitself as distinguished from the bath, the plate itself can contain fromapproximately 0.05 to 1.5 percent phosphorus and from 0 to 2.5 percentnickel by weight, the balance being cobalt.

The nature of the substrate is not critical so long as it isnonmagnetic, conductive and has a smooth surface. Thus metals such asbrass or aluminum can be used as well as nonconductors having aconductive surface which can be deposited ,by vacuum evaporation orelectroless methods. Plastics such as polyesters, polyolefins andcellulose acetate are all suitable.

The following nonlimiting examples illustrate preferred embodiments ofthe invention:

EXAMPLE 1 A film was electrodeposited on a sputtered gold-onpolyestersubstrate from a bath of the following composition:

Cobalt sulfate (hydrate) 200 g/l. Boric acid I 7 l g./l. Sodium chlorideg./l. 5911mm lfluryl 511N816 5 g./l. Sodium hypophosphite 0.3 g./l. NBC!25 g./l. Boric acid l5 g./l. Sodium lauryl sulfate (wetting agent) 5gJl. pH=6.0, adjusted with sodium hydroxide solution. 5 Plating wascarried out at room temperature at a cathode curf rent density of 50ma./in. This films thermomagnetic pro- 3353??? 5-; 52' I 2 2 2 pertiesare plotted in the drawing showing temperature vs Boric acid coerciveforce for a 200 A. thick magnetic film. The plating Sodium laurylsulfate 5 was composed of 99.9 percent cobalt and approximately 0.1 10percent phosphorus by weight. EXAMPLE VII All sulfate hath: COSO47H3O200 g./l. EXAMPLE ll NaH Po,i-|,o 0.3 g./l. Boric acid [5 g./l. Bathcomposition 15 Sodium l yl u a 5 g./l. cosonmo o g./l. H380, l5 g./l.NaC l g.ll. uac n so. s g./l. The characteristic of the platings aregiven in the following NaH,PO-,'H O 0.3 3.11. table: PH=6 #7 V He, HThlck- Rema- 15 0. H 15 C. ness, nence, Example Amons (BH) 150 G. (AB)Ratio A. Br/Bs I Sulfate chloride. 226 88 190 2. 57 100 .864 .do 270 85272 3. 18 200 816 V All chlor1de 245 81 224 3 02 100 850 do 339 94 344 361 200 .833

VI Sulfamate 196 76 170 2 58 100 .881 do 228 73 233 3 12 200 824 VII Allsulfate 210 72 209 2.92 100 .835 d0 r 280 81 290 3. 46 200 825 BH=beforeheating. AH;=al'ter heating for 10 minutes at 150 C. Rat1o=coerclv1ty at15 C. divided by coercivity at 150 C.

Plating was conducted at 5 ma./in. to a thickness of 200 A. on anonmagnetic conductive substrate.

The plating had the following characteristics:

H at 12 445 Oe l-l at 150 1 15 0e H, at 20090 Oe EXAMPLE 111 I Theprocess of example 1 was repeated except that the amount of phosphoruswas changed to 3.0 g./l. NaH PO -H- O and the current density was 500ma./in.

The plating had the following characteristics:

H at 12 294 H at 20061 EXAMPLE IV The process of example 1 was repeatedexcept that the amount of phosphorus was changed to 1.0 g./ 1. NaH PO -HO and there was added 9.5 g./l. NiSO -6 H O. Also the current densitywas 100 ma./in.

The plating had the following characteristics:

l-i at 15095 H at 20063 1n the above examples, the anions present in theplating bath were a combination of sulfate and chloride. However theprocess is not anoin-dependent as the following examples V through Vllshow ln each instance the bath was maintained at 15 C., plating wasconducted at a current density of 200 ma./in. and the pH was adjusted to6.55. Plating thickness was 100 A. and 200 A.

EXAMPLE V Chloride bath: CtiCl:'6H. .O 170 g./l. NaH- -PO:'H O 0.3 g./l.

EXAMPLE VII] The purpose of this example is to illustrate the affect ofa small amount of phosphorus in the composition. Using the generaltechnique of example 1 two platings were made at a pH of 6.52. One bathcontained only cobalt and the other contained in addition 0.35 g./l. ofsodium hypophosphite. Coercivities were measured at 150C. and then at 15C. The following results were obtained:

1 5 C. 150 C. Cobalt alone 165 0e 88 0e Cobalt and phosphorus 334 0c 880e Thus, although the phosphorus had no substantial effect on coercivityat the elevated temperature, it had a very substantial effect oncoercivity at room temperature.

We claim.

1. in a process for thermomagnetic recording of information on arecording medium which isresponsive to heat. wherein the medium isscanned by a heat-producing beam while applying a selected magneticfield thereto while maintaining the heat produced by the beam at anelevated temperature substantially below the Curie temperature toproduce a decrease in coercivity of the medium to render same responsiveto the applied magnetic field and the beam and the applied field definethe means for selectively introducing the information to be recorded,the improvement comprising employing as the medium a plating on anonmagnetic substrate wherein the plating is predominately cobalt withfrom 0.05 to 1.5 percent by weight phosphorus, said plating being in thehexagonal form.

2. The process of claim 1 wherein the coating contains up to 2.5 percentnickel.

3. The process of claim 1 wherein the plating is deposited from a bathmaintained at a pH of from 5.5 to 6.6.

4. The process of claim 1 wherein the plating has a coercivity of atleast Oe and has a ratio of coercivity at 15C. divided by the coercivityat C. of at least 2.

2. The process of claim 1 wherein the coating contains up to 2.5 percentnickel.
 3. The process of claim 1 wherein the plating is deposited froma bath maintained at a pH of from 5.5 to 6.6.
 4. The process of claim 1wherein the plating has a coercivity of at least 100 Oe and has a ratioof coercivity at 15* C. divided by the coercivity at 150* C. of at least2.